The present invention relates to a process for blowing films of compatibilized blends of at least two normally incompatible polymers. More particularly, the present invention relates to compatibilized and melt processible blends of a vinylidene chloride interpolymer and an olefin polymer which are blown as films using the inflated bubble technique.
Vinylidene chloride interpolymers are well known as excellent barriers to mass transport of atmospheric gases and moisture vapor. These interpolymers have limited areas of application, however, because of poor melt processing characteristics. In particular, vinylidene chloride interpolymers in a melt plasticized state have poor heat stability and low melt strength. These same interpolymers, when fabricated, tend to be brittle and to have low impact strength.
Olefin polymers, such as high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and ultra-low density polyethylene (ULDPE), generally have better melt processing characteristics than vinylidene chloride interpolymers. That is, they are melt processible over a wider range of temperatures. The olefin polymers are readily fabricated into articles. They also provide rigidity without brittleness when so fabricated. Notwithstanding such processing advantages, the olefin polymers are excessively permeable to atmospheric gases.
Efforts to combine the best features of vinylidene chloride interpolymers and olefin polymers in a polymer-polymer blend useful in blown films have been unsuccessful until now. Lack of success has been studied by comparing physical properties of the polymer-polymer blend with those of the blend components in a straight line volume fraction relationship (hereinafter referred to as "the rule of mixtures"). The physical properties of the polymer-polymer blends have generally been poorer than those predicted by following the rule of mixtures.
Various explanations have been advanced to explain the aforementioned lack of success. One such explanation was that mixing procedures used to disperse one polymer in a second polymer were inadequate.
It is now believed thatthe lack of success may be attributed to inherent physical incompatibility of vinylidene chloride interpolymers with olefin polymers. A compatibilizer (such as an ethylene copolymer containing as a copolymerized moiety an oxygen-containing monomer) can be used in creating a compatible blend or alloy of vinylidene chloride interpolymers and polyethylenes; it is known that such blends can be easily compression molded into films with good oxygen barrier properties.
EPO Application No. 85111781.2 discloses compatibilized blends, such as described immediately above, which are heat-molded under compression into films having good oxygen barrier properties.
Japanese patent application No. Sho 52-40290, filed Apr. 11, 1977 discloses blends of vinylidene chloride resins and polyolefin type resins or polystyrene type resins which are melt-extruded, then cooled, and after being cooled are biaxially stretched by tentering or by inflation.
It is well-known in the relevant arts that vinylidene chloride polymers and copolymers are lacking in sufficient melt strength to be inflated, while molten, by the bubble blowing technique. For this reason it has been customary to extrude such VCl.sub.2 polymers through a circular die, then after being cooled (frozen), the solidified tube can be tentered or inflated so as to biaxially stretch the film.
It has now been discovered that blends of vinylidene chloride polymers and olefin polymers which have been compatibilized with a compatibilizing polymer can be blown as films using the inflated bubble technique wherein the stretching of the film is performed while the extruded tube of film is still molten, i.e., it is inflated before it has been cooled, if the process is controlled within certain process parameters.